Chapter 2 From Genes to Genomes. 2.1 Introduction We can think about mapping genes and genomes at several levels of resolution: A genetic (or linkage)

Chapter 2Chapter 2

From Genes to Genomes

2.1 IntroductionWe can think about mapping genes and genomes at several levels of resolution:A genetic (or linkage) map identifies the distance between mutations in terms of recombination frequencies. A linkage map can also be constructed by measuring recombination between sites in genomic DNA. A restriction map is constructed by cleaving DNA into fragments with restriction enzymes and measuring the distances between the sites of cleavage. The ultimate map is to determine the sequence of the DNA. From the sequence, we can identify genes and the distances between them.

2.1 Introduction2.1 Introduction 2.2 The nature of interrupted genes2.2 The nature of interrupted genes 2.3 Organization of interrupted genes may be conserved2.3 Organization of interrupted genes may be conserved 2.4 Exon sequences are conserved but introns vary2.4 Exon sequences are conserved but introns vary 2.5 Genes can be isolated by the conservation of exons2.5 Genes can be isolated by the conservation of exons 2.6 Genes show a wide distribution of sizes2.6 Genes show a wide distribution of sizes 2.7 Some DNA sequences code for more than one protei2.7 Some DNA sequences code for more than one protei

nn 2.8 How did interrupted genes evolve?2.8 How did interrupted genes evolve? 2.9 The scope of the paradigm2.9 The scope of the paradigm

bp is an abbreviation for base pairs; distance along DNA is measured in bp.Restriction enzymes recognize specific short sequences of DNA and cleave the duplex (sometimes at target site, sometimes elsewhere, depending on type).Restriction map is a linear array of sites on DNA cleaved by various restriction enzymes.

2.2 Genes can be mapped by restriction cleavage

Figure 2.1 DNA can be cleaved by restriction enzymes into fragments that can be separated by gel electrophoresis.


Figure 2.2 Double digests define the cleavage positions of one enzyme with regard to the other.


Figure 2.3 A restriction map can be constructed by relating the A-fragments and B-fragments through the overlaps seen with double digest fragments.


Figure 2.4 When restriction fragments are identified by their possession of a labeled end, each fragment directly shows the distance of a cutting site from the end. Successive fragments increase in length by the distance between adjacent restriction sites.


Figure 2.5 A restriction map is a linear sequence of sites separated by defined distances on DNA. The map identifies the sites cleaved by enzymes A and B, as defined by the individual fragments produced by the single and double digests.


Haplotype is the particular combination of alleles in a defined region of some chromosome, in effect the genotype in miniature. Originally used to described combinations of MHC alleles, it now may be used to describe particular combinations of RFLPs.SNP (single nucleotide polymorphism) is any site at which a single nucleotide has changed when two (haploid) genomes are compared.

2.3 How variable are individual genomes?

Figure 2.6A point mutation that affects a restriction site is detected by a difference in restriction fragments.


Four alleles for a restriction marker are found in all possible pairwise combinations, and segregate independently at each generation. Photograph kindly provided by Ray White.


Figure 2.7 Restriction site polymorphisms are inherited according to Mendelian rules.

Figure 2.8 A restriction polymorphism can be used as a genetic marker to measure recombination distance from a phenotypic marker (such as eye color). The figure simplifies the situation by showing only the DNA bands corresponding to the allele of the other genome in a diploid.


Figure 2.9 If a restriction marker is associated with a phenotypic characteristic, the restriction site must be located near the gene responsible for the phenotype.


The mutation changing the band that is common in normal people into the band that is common in patients is very closely linked to the disease gene.

Figure 2.9 If a restriction marker is associated with a phenotypic characteristic, the restriction site must be located near the gene responsible for the phenotype.


The mutation changing the band that is common in normal people into the band that is common in patients is very closely linked to the disease gene.

Exon is any segment of an interrupted gene that is represented in the mature RNA product.Intron is a segment of DNA that is transcribed, but removed from within the transcript by splicing together the sequences (exons) on either side of it.RNA splicing is the process of excising the sequences in RNA that correspond to introns, so that the sequences corresponding to exons are connected into a continuous mRNA.Structural gene codes for any RNA or protein product other than a regulator.Transcript is the RNA product produced by copying one strand of DNA. It may require processing to generate mature RNAs.

2.4 Eukaryotic genes are often interrupted

Figure 2.10 Interrupted genes are expressed via a precursor RNA. Introns are removed when the exons are spliced together. The mRNA has only the sequences of the exons.

2.4 Eukaryotic genes are often interrupted

Figure 2.11 Comparison of the restriction maps of cDNA and genomic DNA for mouse -globin shows that the gene has two introns that are not present in the cDNA. The exons can be aligned exactly between cDNA and gene.

2.5 Organization of interrupted genes may be conserved

Figure 2.12 An intron is a sequence present in the gene but absent from the mRNA (here shown in terms of the cDNA sequence). The reading frame is indicated by the alternating open and shaded blocks; note that all three possible reading frames are blocked by termination codons in the intron.


Figure 2.13 All functional globin genes have an interrupted structure with three exons. The lengths indicated in the figure apply to the mammalian -globin genes.


Figure 2.14 Mammalian genes for DHFR have the same relative organization of rather short exons and very long introns, but vary extensively in the lengths of corresponding introns.


Figure 2.15 The sequences of the mouse maj and min globin genes are closely related in coding regions, but differ in the flanking regions and large intron. Data kindly provided by Philip Leder.

2.6 Exon sequences are conserved but introns vary

Zoo blot describes the use of Southern blotting to test the ability of a DNA probe from one species to hybridize with the DNA from the genomes of a variety of other species.

2.7 Genes can be isolated by the conservation of exons

Figure 2.16 Chromosome walking is accomplished by successive hybridizations between overlapping genomic clones.


Figure 2.17 A zoo blot with a probe from the human Y chromosomal gene zfy identifies cross-hybridizing fragments on the sex chromosomes of other mammals and birds. There is one reacting fragment on the Y chromosome and another on the X chromosome. Data kindly provided by Dabid Page.


Figure 2.18 The gene involved in Duchenne muscular dystrophy has been tracked down by chromosome mapping and walking to a region in which deletions can be identified with the occurrence of the disease.


Figure 2.19 The Duchene muscular dystrophy gene has been characterized by zoo blotting, cDNA hybridization, genomic hybridization, and identification of the protein.

2.7 Genes can be isolated by the conservati

on of exons

Figure 2.20 A special splicing vector is used for exon trapping. If an exon is present in the genomic fragment, its sequence will be recovered in the cytoplasmic RNA, but if the genomic fragment consists solely of an intron,


Figure 2.21 Exons coding for proteins are usually short.

2.8 Genes show a wide distribution of sizes

Figure 2.22 Introns in vertebrate genes range from very short to very long.


Figure 2.23 Most genes are uninterrupted in yeast, but most genes are interrupted in flies and mammals. (Uninterrupted genes have only 1 exon, and are totaled in the leftmost column.)


Figure 2.24 Yeast genes are small, but genes in flies and mammals have a dispersed distribution extending to very large sizes.


Figure 2.25 Two proteins can be generated from a single gene by starting (or terminating) expression at different points.

2.9 Some DNA sequences code for more than one protein

Figure 2.26 Two genes may share the same sequence by reading the DNA in different frames.


Figure 2.27 Alternative splicing uses the same pre-mRNA to generate mRNAs that have different combin-ations of exons.


Figure 2.28 Alternative splicing generates the and variants of troponin T.


Figure 2.29 Immunoglobulin light chains and heavy chains are coded by genes whose structures (in their expressed forms) correspond with the distinct domains in the protein. Each protein domain corresponds to an exon; introns are numbered 1-5.

2.10 How did interrupted genes evolve?

Figure 2.30 The LDL receptor gene consists of 18 exons, some of which are related to EGF precursor and some to the C9 blood complement gene. Triangles mark the positions of introns. Only some of the introns in the region related to EGF precursor are identical in position to those in the EGF gene.






Figure 2.31 The exon structure of globin genes corresponds with protein function, but leghemoglobin has an extra intron in the central domain.


Figure 2.32 The rat insulin gene with one intron evolved by losing an intron from an ancestor with two interruptions.


Figure 2.33 Actin genes vary widely in their organization. The sites of introns are indicated in purple; the number identifies the codon interrupted by the intron.


2.11 The scope of the paradigm

Summary

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Chapter 2 From Genes to Genomes. 2.1 Introduction We can think about mapping genes and genomes at several levels of resolution: A genetic (or linkage)